Generally, an electrochemical sensor comprises polarized electrodes (cathode and anode), one of which protrudes at one end of the sensor where it rests against a membrane selectively permeable to the substance to be detected, that is, a membrane that will let pass said substance but will not, or will less readily, let pass the other substances of the fluid to be analyzed. The membrane closes a recess where the end of the sensor is accommodated, and which is filled with an electrolyte. An electrolyte film is inserted between said protruding electrode and the membrane. The other electrode is also in contact with the electrolyte. The outer surface of the membrane is in contact with the fluid sample to be analyzed. The substance to be detected diffuses across the membrane in response to a pressure difference between the two sides of the membrane, and participates in an electrochemical reaction that produces an electrical current between the electrodes. The intensity of this current is a function of the amount of said substance in the fluid.
Such electrochemical sensors require regular upkeep. The electrolyte and the membrane indeed deteriorate with time, and must thus be periodically replaced. While replacing the electrolyte the user must pay particular attention to avoid any contact between his/her skin and the electrolyte. The latter indeed is a caustic product. For this reason, the operation of electrolyte transfer from the supplier bottle to said sensor recess is delicate.